The invention relates to both, a means and a method for controlling the intensity distribution of a laser beam by means of a homogenizing device which is arranged in the radiation path of the laser beam and which superimposes partial beams of the laser beam one another such that a homogenization of the intensity distribution of the laser beam is supported.
Such a means is known from DE 42 20 705 A1. Therein, the intensity distribution of a laser beam is homogenized (spatially equalized) by means of a plurality of lenses arranged in one row perpendicular to the optical axle. These lenses are respectively formed in such a way that individual partial beams of the laser beam superimpose one another so that the laser radiation shown is altogether extensively homogenized.
A continued development of this state of the art is described in DE 196 32 460 C1. Therein, a plurality of illuminating fields each having a homogeneous intensity distribution, are generated, wherein a row of lenses shows several groups of non centric cylinder lens segments (see also U.S. Pat. No. 5,796,521).
The present invention both, requires and assumes knowledge of the afore mentioned state of the art. The patent documents mentioned above are included in the disclosure of the present application by means of reference.
By means of the homogenizing device mentioned especially the radiation of an excimer laser beam is homogenized for several applications, i.e. the intensity distribution of the laser beam is homogenized across its profile. The laser beam emitted (and not yet optically manipulated) by an excimer laser has, as a rule, no homogeneous intensity distribution across its profile. The laser beam emitted by an excimer laser has approximately a rectangular shape in dimensions of roughly 10xc3x9730 mm. In the context of the rectangular shape one speaks of a short axle (meaning e. g. 10 mm long) and of a long axle (meaning e.g. 30 mm long). For a multitude of applications this beam requires to be imaged in an optically extremely reducing fashion. In addition, many applications require also a homogenization of the beam intensity, i.e. a constant intensity distribution of the beam across its profile. Optical devices for the homogenization of laser beams are also known from DE-A-42 20 705 (corresponding to U.S. Pat. No. 5,414,559), DE-38 29 728 A1, DE-38 41 045 A1 and DE 195 20 187 A1.
During the industrial employment of laser radiation, especially excimer laser radiation, applications occur frequently, which not only require a homogenization of the laser radiation impacting on a material to be processed but, in addition, make a purposeful adjustment of the intensity distribution after homogenization desirable. A respective example is the utilization of laser radiation for the production of arrays of holes in a thin foil (e.g. polymer foil). In order to produce arrays of holes in such a foil (typically with a thickness of 20 to 100 xcexc) the laser radiation is directed in a known fashion onto a mask into which arrays of holes are formed. The laser radiation penetrating the mask is imaged onto the substrate (here meaning the foil) by an objective with a reduction of, for example, 2,5 to 5 times. At the same time the intensity of the laser radiation increases square to the factor of the reduction, e.g. at a 5 times reduction the intensity increases 25 times.
In order to produce holes in a foil of the kind mentioned, 100 to 300 pulses of excimer laser radiation, depending on type, are required. In the state of the art laser radiation is homogenized by such means as described in the state of the art mentioned in the beginning.
Nevertheless, it occurs, for example, that during the boring of arrays of holes, as described before, the hole diameters in the respective row are not completely commensurate, especially on the exit side. It is observable that the diameter of She holes arranged on the outside of the row is wider than that of the holes positioned on the inside. This is explained by the absorption of radiation by the particles generated during the ablation process. The concentration of absorbing particles is larger in the center of the track of holes than on the outside. As a result, the outer holes of a track of holes become wider than the inner ones.
Also in other applications can such process-bound influences lead to the effective laser energy being not nearly as homogeneously distributed as originally done so very homogeneously by the homogenizing device.
The object of the invention is to provide an apparatus and a method for the control and adjustment of intensity distribution of laser radiation in a simple way.
The apparatus according to the invention to achieve this object is characterized by a diaphragm placed in the radiation path behind the homogenizing device for cutting the radiation out from the superimposed partial beams in such a way that variable portions are cut out from at least some of those partial beams that have not completely superimposed one another.
The method of control and adjustment according to the invention provides that by means of a diaphragm radiation is cut out from the laser beam in such a way, that variable portions are cut out from at least some of those partial beams that have not completely superimposed one another.
The employment of a diaphragm and a mask, respectively, in connection with a homogenizing device of the kind mentioned herein is known from DE 196 19 481 C1. There, a mask is arranged so far in the rear of the radiation path, however, that the laser beam illuminates homogeneously one aperture in the mask and this homogeneously illuminated aperture is then imaged onto a surface. Therefore, the partial beams are already completely superimposed by one another at the location of the mask and respective equal portions of them are cut out.
The invention relates further to the application of a means and a method for the generation of structures in substrates, especially for the production of arrays of holes with a constant diameter.
A preferred embodiment of the means according to the invention provides that the diaphragm is arranged concentrically with the laser beam. This means that the center of the effective area of the diaphragm coincides at least closely with the axis of the laser beam.
An advantageous embodiment of the invention provides that the diaphragm is adjustable, i.e. the effective area in which the diaphragm cuts radiation out from the laser beam can be altered manually as well as automatically.
Suitable for the invention are apertured diaphragms as well as diaphragms which cut radiation out from the central area of the laser beam and let peripheral beams pass.
Especially suitable are round shaped and more especially circular or oval diaphragms. Angular diaphragms, especially rectangular ones as well as square ones are also suitable.
For the homogenizing-device used in the invention arrays of cylindrical lenses with one convex lens are particularly considered, as described accordingly in the state of the art mentioned at the beginning.
The invention is especially suitable for the homogenization and control of the intensity distribution of an excimer laser beam.